This invention generally relates to interconnecting semiconductor chips. More particularly, it relates to an apparatus for optical communication between chips located on distinct substrate surfaces.
There is a continuing effort to increase the speed of electronic circuitry, both by increasing the speed of the devices themselves and by shortening the electrical paths between devices providing more circuits on a single chip and more chips on a single substrate. However, despite these efforts at miniaturization, as electronic components have become faster, the speed of electronic equipment, such as a computer, is increasingly limited by the time required for electronic signals to travel from one component to another within the machine and by the broadening of electric pulses as the signals travel in conductive media.
To speed signaling, there is an increased desire to replace electronic signals with optical signals for communication among components. Light is not affected by transmission line delay, by resistance-capacitance delay, or by inductance delay. Thus, the transit time between components is reduced. Also, because the signal shape suffers little degradation, the data rate is significantly higher for optical communication than for electronic communication. For example, for short range communication, the data rate is limited to about 70 to 85 Mb/s in aluminum conductors; using copper conductors, speed can be boosted to about 100 to 125 Mb/s. Yet optical transmission currently allows data rates of about 20 Gb/s, 200 times faster.
Optical signal transmission has other advantages as well, including immunity to electromagnetic interference and electromagnetic pulses which can disrupt electrical transmission, as well as electrical isolation, reduced power consumption, and higher reliability because of fewer electrical contacts.
In the prior art approaches to optical communication, the optical signal is generally carried in an optical fiber or waveguide. Optical fiber has won widespread acceptance in part because light energy can, at a high data rate, be efficiently coupled into or out of the optical fiber, transmitted in any direction and around any obstacles, split into several signals, and carried for long distances with little dispersion or attenuation. Optical fiber possesses clear advantages in medium- and long-range optical communication. Optical fiber links interconnecting computer processors and peripherals are commercially available. However, for very short-range communication, as attenuation of the signal is not an important factor, the use of optical fiber is not particularly advantageous because of the difficulty and complexity in making optical fiber connections at small dimensions.
Surface waveguide has been introduced for short-range communication between semiconductor chips on the same substrate surface. Waveguide can be fabricated on the substrate surface using many of the same lithography techniques used in semiconductor fabrication, and therefore, the optical connections are much more easily integrated into the substrate.
One way of arranging a plurality of substrates bearing semiconductor chips in a minimum space would be in a closely spaced, stacked assembly. Electrical bus structures could provide power and communication between the substrates. However, neither surface waveguides nor optical fibers provide the optimum means of optical communication between the substrates. For communication between distinct substrate surfaces, a surface waveguide is not a viable solution as it would find no mechanical support. Interconnecting a closely spaced stack of substrates with a plurality of optical fibers connected to different substrates would pose severe mechanical problems and may add enormous and unnecessary complexity and cost.
The present invention proposes the use of free space optical communication to solve these problems. The term free space generally includes gaseous media like air, which have an index of refraction approximately equal to 1.
Free space optical communication is known in the art, arguably being presaged by the smoke signals of the American West. Free space has been cited in the literature as advantageous for short distance data communication among adjacent computers. Where there are no obstacles in the optical path, free space optical communication offers a far simpler and cheaper signal medium. In addition, light travels 50% more slowly in either fiber or surface wave guide than in free space. However, in a closely spaced stack of substrates, each intervening substrate presents an obstacle to optical communication between nonadjacent, nonfacing substrate surfaces. Optical transmitters and receivers could be placed at the edges of the substrates, but as substrates get large, communication would be increasingly limited by the time required for the electrical signals to travel from chip to edge of the substrates.